The presently claimed invention relates to an alumina coated cutting tool for chipforming machining.
Chemical Vapor Deposition (CVD) of alumina on cutting tools has been an industrial practice for more than 15 years. The wear properties of Al.sub.2 O.sub.3 as well as of other refractory materials have been discussed extensively in the literature.
The CVD-technique has also been used to produce coatings of other metal oxides, carbides and nitrides, the metal being selected from transition metals of the IVB, VB and VIB groups of the Periodic Table. Many of these compounds have found practical applications as wear resistant or protective coatings, but few have received as much attention as TiC, TiN and Al.sub.2 O.sub.3.
Cemented carbide cutting tools coated with various types of Al.sub.2 O.sub.3 -coatings, e.g., pure .kappa.-AL.sub.2 O.sub.3, mixtures of .kappa.- and .alpha.-Al.sub.2 O.sub.3 and very coarse-grained .alpha.-Al.sub.2 O.sub.3 have been commercially available for many years. Al.sub.2 O.sub.3 crystallizes in several different phases: .alpha., .kappa., .gamma., .beta., .theta., etc. The two most frequently occurring phases in CVD of wear resistant Al.sub.2 O.sub.3 -coatings are the thermodynamically stable, hexagonal .alpha.-phase and the metastable .kappa.-phase. Generally, the .kappa.-phase is fine-grained with a grain size in the range 0.5-2.0 .mu.m and often exhibits a columnar coating morphology. Furthermore, .kappa.-Al.sub.2 O.sub.3 coatings are free from crystallographic defects and free from micropores or voids.
The .alpha.-Al.sub.2 O.sub.3 grains are usually coarser with a grain size of 1-6 .mu.m depending upon the deposition conditions. Porosity and crystallographic defects are in this case more common.
Often, both .alpha.- and .kappa.-phase are present in a CVD alumina coating deposited onto a cutting tool. In commercial cutting tools, Al.sub.2 O.sub.3 is always applied on TiC coated carbide or ceramic substrates (see, e.g., U.S. Pat. No. 3,837,896, now Reissue U.S. Pat. No. 29,420) and therefore the interfacial chemical reactions between the TiC-surface and the alumina coating are of particular importance. In this context, the TiC layer should also be understood to include layers having the formula TiC.sub.x N.sub.y O.sub.z in which the carbon in TiC is completely or partly substituted by oxygen and/or nitrogen.
The practice of coating cemented carbide cutting tools with oxides to further increase their wear resistance is in itself well-known as is evidenced in e.g., U.S. Pat. Reissue No. 29,420 and U.S. Pat. Nos. 4,399,168; 4,018,631; 4,490,191 and 4,463,033. These patents disclose oxide coated bodies and how different pretreatments, e.g., of TiC-coated cemented carbide, enhance the adherence of the subsequently deposited oxide layer. Alumina coated bodies are further disclosed in U.S. Pat. Nos. 3,736,107; 5,071,696 and 5,137,774 wherein the Al.sub.2 O.sub.3 layers comprise .alpha., .kappa. and respective .alpha.+.kappa. combinations.
U.S. Pat. No. 4,619,866 describes a method for producing fast growing Al.sub.2 O.sub.3 layers by utilizing a hydrolysis reaction of a metal halide under the influence of a dopant selected from the groups consisting of sulphur, selenium, tellurium, phosphorous, arsenic, antimony, bismuth and mixtures thereof. A few examples of sulphur-based dopants are H.sub.2 S, COS, CS.sub.2, SF.sub.6, SF.sub.4, SO.sub.2 Cl.sub.2 and SO.sub.2. Under these process conditions, essentially two phases of Al.sub.2 O.sub.3, the .alpha.- and .kappa.-phases, are produced. The resulting coating consists of a mixture of the smaller .kappa.-grains and the larger .alpha.-grains. The process yields coatings with an even layer thickness distribution around the coated body.
U.S. Ser. No. 08/238,341 (our reference: 024000-881) discloses a method of growing a fine-grained .kappa.-alumina coated.
U.S. Ser. No. 08/159,217 (our reference: 024000-993) discloses a method for obtaining a fine-grained, (012)-textured a-Al.sub.2 O.sub.3 coating. This particular Al.sub.2 O.sub.3 -coating applied on cemented carbide tools has been found particularly useful for cast-iron machining.
Due to the difference in thermal expansion coefficient of alumina and a cemented carbide substrate, cooling cracks, forming an interconnected network, are frequently present in an Al.sub.2 O.sub.3 -coating, see, e.g., U.S. Pat. No. 5,123,934, FIG. 4 herein. During certain cutting operations with Al.sub.2 O.sub.3 -coated tools, the cooling cracks propagate into the cemented carbide substrate and cause premature tool failure. The cooling cracks may also initiate spot-wise flaking of the entire coating structure.